Aerogels are a diverse class of solid, porous materials exhibiting an uncanny assemblage of extreme material properties and extremely light weight.
Have you ever dreamt of sleeping on fluffy white clouds or diving in a pool of solid air, only to swim in its strange sheer mushy texture. Aerogels do look a lot like your imagination wool. However, they are very different. Being the lightest material in existence, up to now, aerogels are incredibly strong and resistant to a wide range of harsh conditions.
A piece of aerogel balanced on the nails of the hand, depicting its lightweight nature (Photo Credit: NASA/Wikimedia Commons)
“Aerogel” cannot be considered as a specific material or mineral, such as cotton or graphene, and have a specific chemical formula. Instead, they are a diverse class of solid porous materials exhibiting an uncanny assemblage of extreme material properties, consisting of a group of materials with a particular geometrical structure—an extremely porous solid foam with a high linkage between branched structures across the material. These linkages, although spanning a few nanometers, are incredibly strong and durable. Surprisingly, these “mystically flamboyant” materials have been present throughout history for a longer period than you may think. American chemistry professor Samuel Kistler, in 1931, published his initial discoveries about this material after successfully inventing it, a process that involved a lot of experimentation.
How To Make Aerogel?
Imagine preparing a bowl of a sweet gelatin dessert. The process for preparing aerogel is actually quite similar. The gelatin powder is mixed in hot water and then cooled in a refrigerator. What you get is a gel. At this point, the aerogel and your regular edible jello are no different. If you placed this wiggly gel in an oven now and removed all of its moisture, your jelly would undoubtedly turn to powder. This is because, when the moisture is being siphoned out as steam, the structural bonds between the jelly material are pulled inwards, leaving behind nothing but dust.
However, if you somehow managed to remove all the liquid content of the gel, without damaging the solid structure and shape, what you would be left with is a low-density, extremely porous solid. This is precisely how aerogels are made.
So, how do you remove the liquid without damaging the solid? Here’s a video demonstrating a DIY aerogel.
The Solution: Supercritical Drying
Supercritical drying is a complex method in which liquid is removed through capillary action. Every pure substance (that does not decompose) has a critical point, which is a specific pressure and temperature at which the distinction between the liquid state and gas phase disappears. This phase is referred to as a supercritical fluid.
To produce the aerogel, a sealed container with a liquid (mostly silica) is taken below its critical point. The container is equipped with a pressure gauge on top and tools to increase the temperature. Some of the liquid evaporates until the vapor pressure of the liquid and the pressure in the container become equal. If the container is then heated, the pressure inside increases due to the vapor pressure rising with temperature. As the critical point of the liquid approaches, the pressure in the container compresses the vapor molecules close together, resulting in the vapor becoming nearly as dense as a liquid.
As the temperature in the container rises, the molecules in the liquid gain enough kinetic energy to overcome the attractive forces that hold them together. Eventually, the critical point is reached, causing the meniscus between the two phases to blur and creating a single supercritical phase. In this phase, the surface tension in the fluid gradually decreases to zero, resulting in a decrease in capillary stress.
The supercritical fluid now fills the entire vessel and its pores are filled with gel. The liquid in the gel can be removed without hindrance from surface tension by partially depressurizing the vessel, as long as the pressure remains above the critical pressure. The temperature of the container must also remain above the critical temperature.
The goal is to remove enough fluid from the vessel while it is still in the supercritical state. This ensures that when the vessel is fully depressurized below the critical point, there will be no substance left to condense. Once enough fluid has been removed, the vessel is slowly depressurized and cooled to ambient conditions. As this happens, the remaining fluid in the vessel returns to a gaseous state as it passes back through the critical point. The liquid in the gel is completely converted to gas, without any capillary stress, leaving behind the aerogel.
There are three main types of aerogels: silica, carbon, and metal oxides. These aerogels have unique structural and chemical properties, making them useful in a variety of modern equipment. Silica aerogels, in particular, are commonly discussed due to their translucent blue color caused by the scattering of blue light.
Carbon-based aerogels, on the other hand, have a greyish black color and a charcoal-like texture. Despite their unappealing appearance, they have high electrical conductivity and are used in fuel cells, desalination systems, and supercapacitors. Metal oxide aerogels, as the name suggests, are made from metallic oxides and have their own unique properties. They are used as catalysts, explosive matrices, and antecedents for other materials.
Aerogels have various applications due to their low thermal conductivity and lightweight nature. They are used in building construction, appliance insulation, storage media, automobiles, space vehicles, solar devices, and solar ponds.
Aerogel blankets are utilized to safeguard critical systems from the extremely cold hydrogen fuel used for launching NASA’s space shuttles. These blankets have a high porosity and low density, making them suitable for various applications such as catalysis, machine sensors, fuel storage, ion exchangers, exhaust filters, pigment carriers, and templates. They are also used as light guides due to their mild translucency and low refractive index, and find application in lightweight optics.
Additionally, aerogel blankets are acoustically opaque, making them ideal for lining the walls of soundproof rooms and for use in ultrasonic distance sensors. Their lightweight and mild elasticity make them effective energy absorbers in hypervelocity particle traps. Furthermore, aerogels have a high surface area and low dielectric constants, making them commonly used in dielectrics for ICs and capacitors because of their high dielectric strength.
As evident, this distinctive category of substance has the capability to perform various tasks beyond preventing moisture from entering a shoe container!